just some little things,

first don't be afraid to spread that center section a little, I understand you got to design so people with 800x600 can see the site correctly
but when people with 1024x768 or higher comes they see a site in the middle and 2 big gap on each side.
Make that <table width="%"> instead of a fixed size. Then you could always make the menu table a fixed width for the design.

second, any site should be navigable without javascript, that means the menu button should be clickable and the sub menu should appear in the left section.
You could verify if the client browser support javascript and do the thing accordingly. (Personally little pop-down menu annoy me)

third, you should have a printer friendly button on each page. one that only print the content of a section and not the whole page with the menu and all.
you can do it really easily using some php.
I.E. everything from the t-case section could be put in tcase.php
then put each sub-section in separate functions and when you want to print a certain sub-section you call the function corresponding to it.
So when you want a printer friendly version, you open a new window and just call the function to print only what is usefull.

If you want some help with php I can put some example here that will do what I'm saying.

I've studied some thing about website ergonomics and the fixed width thing is one of your bigest mistake and it's one that many people make.

Overall it look good.